1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a liquid ejection head for ejecting liquid droplets.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head (liquid ejection head) for ejecting droplets of ink.
As for the recording heads used in these liquid-ejection-type image forming apparatuses, several different types are known. One example is a piezoelectric recording head that ejects droplets by deforming a diaphragm using, e.g., piezoelectric actuators. When the piezoelectric actuators deform the diaphragm, the volumes of chambers containing the liquid change. As a result, the internal pressures of the chambers increase, thus ejecting droplets from the head. Another example is a thermal recording head that ejects droplets by increasing the internal pressures of chambers using, e.g., heaters disposed in the chambers. The heaters are heated by electric current to generate bubbles in the chambers. As a result, the internal pressures of the chambers increase, thus ejecting droplets from the head.
For such liquid-ejection type image forming apparatuses, there is demand for enhancing throughput, i.e., speed of image formation. One way to increase the throughput is to enhance the efficiency of liquid supply. For example, a tube supply method is proposed in which ink is supplied from a large-volume ink cartridge (main tank) set in an apparatus body to a head tank (sub tank or buffer tank) mounted on an upper portion of the recording head through a tube.
Such a tube supply method can reduce the weight and size of a carriage section mounting the recording head or the head tank, thus reducing the size of an entire apparatus including a structural system and a driving system.
However, to further enhance printing throughput, an increase in the number of nozzles of a recording head, an increase in the flow amount of ink feeding associated with use of higher frequency in driving a recording head, and an increase in viscosity of ink associated with shortening of drying time may be advanced. As a result, a pressure loss due to a fluid resistance of a tube against a flow of ink may cause an inks supply shortage. In particular, for an apparatus to record large-size print media, a long tube generates a large pressure loss and is more likely to cause a failure.
Conventionally, for example, JP-4032953-B (JP-2004-142405-A) proposes an apparatus having a differential pressure valve at an upstream side of an ink supply route to supply ink when a negative pressure in a sub tank is greater than a predetermined pressure value. To enhance a performance of discharging bubbles from the sub tank, the apparatus also has a mechanical assembly to forcefully open the differential pressure valve and perform choke cleaning.
JP-2007-216535-A or JP-2010-120340-A proposes to provide a float valve in a head tank. When air is exhausted from the head tank, an ink level rises. As a result, a float closes an exhaust passage to discharge only air.
However, in a configuration described in JP-4032953-B (JP-2004-142405-A), air mixed in an ink supply route can be discharged only by choke cleaning, thus resulting in an insufficient bubble discharge performance. In addition, a relatively large amount of ink is discharged with bubbles, thus causing wasteful ink consumption.
In a configuration described in JP-2007-216535-A, closing of the exhaust passage relies on a retaining force of a meniscus in an opening portion opened to an outside of the exhaust passage. As a result, if the meniscus is broken by some factors, the interior of the head tank may turn into atmospheric pressure, thus causing a failure, such as ink leakage from nozzles of a recording head.
An ink supply system described in JP-2010-120340-A, basically creates a negative pressure in the head tank by a liquid level difference, thus causing a challenge in an increase in exhausting speed. In other words, if exhausting speed is increased, a negative pressure in the head tank increases, thus sucking air from nozzles of a recording head into the head tank.